Camera flash with reconfigurable emission spectrum

ABSTRACT

A method and an apparatus for spectrum synthesis for use in a flash unit, wherein the spectrum synthesis includes combining a plurality of emissive light sources in order to provide a combine output beam and producing the output spectrum for the combined output beam at least based on a reference spectrum. The reference spectrum can be obtained by sensing the spectrum of ambient light or selected from a plurality of stored spectra. The flash unit has at least two emissive light sources and each of the light sources can be adjusted relative to each other so that the outputs from the light sources can mimic a selected illumination scenario. It is possible to use a mixture of quantum dots to tailor each light source so that the combined spectra from different light sources can reasonably mimic a number of frequently used illumination scenario.

CROSS-REFERENCE AND RELATED APPLICATIONS

This patent application is a continuation application of U.S. Pat. No.8,598,798, issued on Dec. 3, 2013 (U.S. patent application Ser. No.12/322,709 filed on Feb. 4, 2009), which claims benefit to U.S. PatentApplication Ser. No. 61/072,151 filed on Mar. 28, 2008.

BACKGROUND

The disclosure relates generally to illumination for both photographyand general lighting and, in particular, to camera flash. Anillumination source is largely characterized by luminous flux (lumen)and spectral power distribution (W/nm). The former is a metric of theperceived brightness whereas the latter determines the color of thelight via multiplication with the color matching functions. The color ofwhite light can be expressed both by CIE chromaticity coordinates and bythe correlated color temperature (CCT), that is, the temperature of ablack-body radiator resulting in a spectrum which, when multiplied bythe color matching functions, yields the same color as the originalillumination source. For example, an incandescent light bulb has aspectrum corresponding to a CCT of 3200° K whereas a Xenon camera flashtypically has a CCT of 9000° K. The CCT of daylight varies by weather,location and time of the day and year.

The human vision adapts to the illumination so an object with flatreflection spectrum looks white under many different illuminationsources. In contrast, a film-based camera is not able to adapt. In adigital camera, the sensor usually has fixed RGB (red, green, blue)filters, post-processing of the raw image data can be used to adjust thewhite balance to a predefined value, usually expressed in CCT. Inparticular, in consumer cameras, this process is automated via automaticwhite balancing (AWB) algorithms, i.e. the white point of the image isadjusted after it has been recorded. These algorithms are often veryintricate and advanced but the result is always implemented by adjustingthe relative gain in the red, green, and blue channels.

SUMMARY

A method and an apparatus for spectrum synthesis for use in a flash unitare provided. The spectrum synthesis comprises combining a plurality ofemissive light sources in order to provide a combine output beam andproducing the output spectrum for the combined output beam at leastbased on a reference spectrum. The reference spectrum can be obtained bysensing the spectrum of ambient light or selected from a plurality ofstored spectra. It is possible that a user can determine the type ofambient light source and select the reference spectrum based on thedetermined type. The flash unit has at least two emissive light sourcesand each of the light sources can be adjusted relative to each other sothat the outputs from the light sources can mimic a selectedillumination scenario. When the number of the light sources in the flashunit is too small, the difference between the synthesized spectrum andthe spectrum of the selected illumination scenario can be significant.It is possible to use a mixture of quantum dots to tailor each lightsource so that the combined spectra from different light sources canreasonably mimic a number of frequently used illumination scenario. Ingeneral, the difference between the synthesized spectrum and thespectrum of the selected illumination scenario can be reduced byincreasing the number of the light sources in a flash unit. In any case,the minimum number of the light sources is two.

Thus, in accordance with the various aspects of the invention, a methodfor spectral synthesis is disclosed. According to one aspect, the methodincludes providing at least a first emissive light source and a secondemissive light source for a camera flash, wherein the first emissivelight source is configured for producing a first light output with afirst spectral distribution, and the second emissive light source isconfigured for producing a second light output with a second spectraldistribution different from the first spectral distribution; andproviding electrical access to the first and second emissive lightsources such that at least the first light output is adjustable relativeto the second light output for producing a combined light output with athird spectral distribution.

Another aspect is a flash module. According to one embodiment, the flashunit includes at least a first emissive light source configured forproducing a first light output with a first spectral distribution; and asecond emissive light source configured for producing a second lightoutput with a second spectral distribution different from the firstspectral distribution, wherein at least the first light output isadjustable relative to the second light output for producing a combinedlight output with a third spectral distribution for a camera flash. Theadjustment of the light output can be achieved by controlling theamplitude of the electrical current or by controlling the pulse-width ina pulse-width modulated current.

Another aspect is a stand-alone camera or a camera in an electronicdevice such as a mobile phone, the camera having a flash unit, whereinthe flash unit includes at least a first emissive light sourceconfigured for producing a first light output with a first spectraldistribution; and a second emissive light source configured forproducing a second light output with a second spectral distributiondifferent from the first spectral distribution, wherein at least thefirst light output is adjustable relative to the second light output forproducing a combined light output with a third spectral distribution fora camera flash. The camera can be a digital camera or a film-basedcamera.

Various embodiments will become apparent upon reading the description ofthe drawings taken in conjunction with FIGS. 1 to 7.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a block diagram showing a digital camera.

FIG. 1 b is a block diagram showing a film-based camera.

FIGS. 2 a-2 d show various illumination modules.

FIGS. 3 a-3 b show emissive light sources.

FIG. 4 shows a flash unit.

FIG. 5 shows an example of synthesized flat spectra from the output of20 LEDs.

FIG. 6 shows an example of an arbitrary synthesized spectra from theoutput of 20 LEDs

FIG. 7 is a flowchart for minimizing the spectral error of thesynthesized spectrum.

DETAILED DESCRIPTION

The optimization of the transmission spectra of the color filters in adigital camera assumes a standard illuminant, standard objectreflectance spectrum, standard object preference, or a combinationthereof. This means that the camera is, from a sensitivity point ofview, sub-optimized for most illumination sources, especially for cameraflash light. In order to perform AWB, the gain factors need to beadjusted for each R, G, and B channel to achieve the desired whitebalance (CCT). Increased gain inevitably results in larger image noiseand a grainier image.

If the photograph is taken in the illumination for which the sensor isspectrally optimized, gain can be minimized. In most situations,however, the illumination is a superposition of several sources so theresultant spectrum, and hence the CCT, is not well defined across theimage. This situation occurs, for example, indoors close to a windowwhere the object is illuminated by both daylight and artificial light(incandescent bulbs, fluorescent lamps, light-emitting diode (LED) lampsetc). The same problem occurs when using a conventional flash in ambientlight.

A problem with both LED and Xenon flashes is the low color renderingindex (CRI) which is caused by their discontinuous emission spectrum(pseudo-white LEDs have only blue and yellow light). This results inmissing colors or depth when taking photographs or poor light qualitywhen the mobile phone flash is used in torch mode.

Image noise generated by gain is typically reduced by low-pass or otherfiltering in the image-processing chain of the camera but this leads toartifacts and reduced sharpness.

The problem of mixed illumination sources and a white point CCT varyingwithin the image has been solved by applying filters to one or severalof the sources. For example, a blue filter can be attached toincandescent lamps to give the same CCT as daylight. However, thespectra are still not identical which can confuse the AWB algorithm.Also, it is not practical to carry and attach blue filters every time aphoto is to be taken. AWB algorithms are not perfect and objects withextreme color distributions often appear with the wrong color balance.If instead the actual illumination spectrum can be identified, moreaccurate white balancing can be achieved.

Accordingly, in one embodiment, the first step is to determine the typeof ambient light source. The subsequent step is to adjust the spectrumof the flash so that the spectrum and, therefore, the CCT coincide withthat of the ambient light. When shooting occurs in darkness or in dimambient light where the majority of the illumination comes from theflash, the flash spectrum is adjusted to that of the camera/film toachieve maximum camera/film speed. The synthesized/identified spectrumis fed back to the AWB algorithm to achieve the actual white pointwithout analyzing the image or adjusting the RGB gain factors whichleads to reduced color artifacts. When used in the torch mode of theflash or in general lighting, a spectrum of any light source can besynthesized. This is useful for accurate rendering of surface colors.

Synthesizing an arbitrary flash spectrum can be accomplished bycombining two or more individually addressable LEDs with differentspectra. The modulation of each LED is done either by current, pulsewidth, or a combination thereof. The emission could originate eitherfrom the LED itself, LED+broadband phosphor, or LED+any photoluminescentmaterial, including quantum dots (QD), which allow precise spectraldesign over the entire visible range when combined with deep purple orUV exciting LEDs. The emission can also solely be originated from thephotoluminescent material. The emission spectrum can be simply aGaussian distribution with the peak wavelength determined only by the QDsize. Typical full-widths at half-maximum (FWHM) are 10-15 nm and peakwavelength controllability within +/−1 nm. By mixing QDs of severalsizes and tuning their number ratios, a tailored spectrum for each LEDcan be obtained.

Another way to synthesize the flash spectrum is using a fast spatiallight modulator (SLM) below which QDs or mixtures of QDs correspondingto the different spectra are printed. The QDs can then be excited by asingle LED and the duty of each wavelength is controlled by the SLM.SLMs with microsecond response can be implemented with bothferroelectric liquid crystals (FLCs) and micro electro-mechanicalsystems (MEMS). In both cases, a rotationally symmetric structure andseparate Fresnel lenses for each emitting region is required todistribute the light from each emitter uniformly.

To reproduce the approximate spectra of all possible illuminationscenarios (sunlight, cloudy sky, tungsten lamp, fluorescent light, etc),the fluent weights of the base spectra are iteratively adjusted untilthe difference between the illuminant spectrum and the synthesizedspectrum converges to a minimum. When the base spectral weights

(current and/or pulse widths) have been determined, they are stored in alook-up table (LUT) along with an illuminant look-up index.

The illuminant indices are determined from the spectrally calibratedsensor response. The sensor could be a spectrometer, trichromaticsensor, or a two-channel sensor (see Table 1), or the camera itself.

The LUT also contains pre-calculated CCTs of the light sources which arefed into the white-balancing algorithm (this already uses CCT as aninput parameter). In this way, color artifacts from determining thewhite point via image analysis can be avoided.

TABLE 1 Example of Relative Response from A Two-Channel Ambient LightSensor LIGHT SOURCE CCT CHANNEL RATIO F11 (Fluorescent lamp) 3700K0.127860953 F12 (Fluorescent lamp) 2800K 0.133593675 Daylight simulator5600K 0.100576456 Halogen lamp 2600K 0.567822155

In order to minimize the risk for temporal color artifacts in scanningsensor systems, e.g. CMOS sensors, current modulation is preferred overtemporal modulation. It is also possible to distribute energy of theshortest pulse throughout a burst of multiple pulses, the width of whichcorresponds to the longest pulse width of all emitters.

In sum, according to various embodiments, the spectrum of a flash issynthesized by using two or more emissive light sources, at least one ofthe light sources has a different spectrum from the other. The flash canbe used in a digital camera, a film-based camera, or a separate unit. Ablock diagram of a digital camera, according to one embodiment of theinvention is shown in FIG. 1 a.

As shown in FIG. 1 a, the digital camera 1 has a single lens or a lenssystem 10 for forming an image on a sensor 20, such as a solid-statesensor. Under the control of a processor 30, an image is captured by theuser taking a picture. The captured image can be stored in a memory 40.The camera 1 also has a flash unit 50 with at least two emissive lightsources 52 and 54 for emitting lights with different spectral ranges ordistributions. The flash unit 50 is operatively connected to a controlmodule 60 so that the light sources 52 and 54 can be separatelycontrolled or addressed by the control module 60. An LUT is operativelyconnected to the control module 60 and the processor 30. The camera 1also has a user-interface 80 to allow a user to choose the settings ofthe camera, including the choice of illumination scenarios. The camera 1may have a light spectrum sensing unit 90 for determining the spectraldistribution of ambient light, for example. The sensing unit 90typically has a diffusing light collecting lens to average theillumination from many different directions. Alternatively, the cameraitself is used as a light sensor by temporarily defocusing to achievethe same effect. The sensing unit 90 is calibrated against all possibleillumination sources and the calibration data is written into the LUT.The signals from the sensing unit for an arbitrary illumination are thencompared to the LUT, the corresponding base spectrum weighs are loaded,and the flash is driven with the corresponding weighs. If the sensorsignal is below a pre-defined value, the flash unit is identified as themain illumination, and the emitter weights are selected to produce aspectrum corresponding to the maximum spectral sensitivity of thesensing unit.

A film-based camera is shown in FIG. 1 b. As shown in FIG. 1 b, thecamera 1 allows a section of a photographic film 22 to be placed at theimage plane of the lens or lens system 10 for recording an image. Ashutter 24, under the control of the processor 30 and a shutter releasedriver 26, is used to control the exposure on the film.

In spectrum synthesis, the output of the light sources 52 and 54 iscontrolled by the electrical current. As shown in FIG. 2 a, the controlunit 60 has at least a first current source 62 and a second currentsource 64 to separately provide electrical current 112 and electricalcurrent 114 to the light sources 52 and 54. The electrical power sourcecan be a battery 160 or a transformer connected to another power source,such as an electrical outlet. According to another embodiment, theoutput of the light sources 52 and 54 is controlled by the pulse-widthof two pulse-width modulation power supplies 66 and 68, as shown in FIG.2 b. The current 112′ and the current 114′, as shown in FIG. 2 b, arepulse-width modulated currents.

According to yet another embodiment, the flash unit 50 has a battery 160and the current sources 62 and 64 to provide electrical current to thelight sources 52 and 54, as shown in FIG. 2 c. The flash unit 50 alsohas electrical connectors for receiving control signals 116 and 118 froman external controlling processor 61 so as to control the output of oneor both of the light sources.

According to a different embodiment, the flash unit 50 does not includea battery 160. Instead, the flash unit 50 has electrical connectors 162for connecting to an external battery.

FIG. 3 a shows an emissive light source for use in the flash unit 50. Asshown, the emissive light source 52, 54 has a light-emitting diodeencased in a transparent body with a lens for beam formingAlternatively, an optically excitable material is placed between thebeam forming lens and the light-emitting diode so that the material canbe used as a secondary emissive source, as shown in FIG. 3 b. Forexample, the optically excitable material can be a broadband phosphor ora photoluminescent material, including quantum dots. The light-emittingdiode in this arrangement, can be the diode that emits light in the deeppurple or UV. The output of the light source as shown in FIG. 3 a andFIG. 3 b can be controlled by the input current to the light emittingdiode as illustrated in FIG. 2 a. Alternatively, the output iscontrolled by pulse width modulation, as illustrated in FIG. 2 b.

FIG. 4 shows an emissive light source, according to another embodiment.As shown in FIG. 4, the flash unit 10 may comprise one or more excitinglight emitting diodes, a plurality of quantum dots arranged in an array,and one or more spatial light modulators placed between the lightemitting diodes and the quantum dots in order to control the lightoutput from the quantum dots. For example, the UV/NUV light emittingdiodes can be used to excite single-size quantum dots, each of whichgives a Gaussian or near-Gaussian spectrum. It is also possible that amixture of quantum dots are used to produce a combined spectrum with aparticular spectral distribution, for example.

FIG. 5 shows an example of synthesized flat spectra from the output of20 LEDs, wherein each of the LEDs produces a Gaussian or near-Gaussianspectrum of a different wavelength.

FIG. 6 shows an example of an arbitrary synthesized spectra from theoutput of 20 LEDs.

FIG. 7 is a flowchart illustrating an exemplary procedure in determiningthe synthesized spectrum, according to one embodiment. As shown in theflowchart 300, the goal is to obtain a synthesized spectrum S′ (λ) inreference to an illuminant or source spectrum S(λ). At step 301, thesource spectrum S(λ) is obtained from the sensing unit 90 or retrievedfrom the LUT 70 in the camera (see FIGS. 1 a and 1 b). For simplicity,it is assumed that the source spectrum is normalized such that its peakis set equal to 1. If the number of light sources in the flash unit isn, then the synthesized spectrum is S′(λ) which is expressed as the sumof W_(n)S_(n)(λ), with S_(n)(λ) being the base spectra of the lightsources, and W_(n) being the fluent weights. At step 302, each fluentweight W_(n) is set equal to 1. At step 303, the wavelength λ_(p) atwhich the spectral power distribution of S(λ) reaches a maximum isdetermined, either from measurement or from the LUT. At step 304, thesynthesized spectrum S′(λ) is normalized to become S′(λ_(p)), in eachiteration, so that the peak in the normalized synthesized spectrumS′(λ_(p)) is equal to 1. During the normalization process at step 304,the weight W_(n) of each of the base spectra is adjusted to W_(n)′. Atstep 305, the relative error ε=S′(λ_(n))/S(λ_(n)) for each base spectrumis computed, where λ_(n) is the peak wavelength of that base spectrum.At step 306, the weight W_(n)′ is adjusted based on the relative error εso that the error vanishes after the adjustment. After the weight W_(n)′of each of the base spectrum has been adjusted, as determined at step307, an interim synthesized spectrum is computed at step 308. Since thebase spectra have finite distributions, there will be errors for otherwavelengths in each base spectrum. These errors may be minimized byiteration. At step 309, if it is determined that the sum of errors hasreached a predetermined value, the interim synthesized spectrum is usedas the final synthesized spectrum. The weight W_(n)′ for each basespectrum can be used to adjust the output of the light source.

It should be noted that when a particular illuminant spectrum S(λ) isstored in the LUT and the base spectra of the light sources in the flashunit are known, it is possible to store the fluent weights for the basespectra in the LUT once a synthesized spectrum is determined. Forexample, once a synthesized spectrum of a candle-lit scenario has beendetermined according to the base spectrum of the light sources in theflash unit, the fluent weights for this particular synthesized spectrumcan be stored in the camera. If the user chooses to take a picture withthis synthesized candle-lit spectrum through the user interface 80 (seeFIGS. 1 a and 1 b), the control module 60 will adjust the output of thelight sources in the flash unit 50 using the stored fluent weights inthe LUT 70, for example.

In sum, a method and an apparatus for spectrum synthesis for use in aflash unit are provided. The flash unit has at least two emissive lightsources and each of the light sources can be adjusted relative to eachother so that the outputs from the light sources can mimic a selectedillumination scenario. The emissive light sources can be LEDs or otheradjustable light sources, or a combination thereof. Furthermore, one ormore non-adjustable light sources, such as Xenon flash lights, can beused in combination with one or more adjustable light sources in a flashunit. When the number of the light sources in the flash unit is toosmall, the difference between the synthesized spectrum and the spectrumof the selected illumination scenario can be significant. It is possibleto use a mixture of quantum dots to tailor each light source so that thecombined spectra from different light sources can reasonably mimic anumber of frequently used illumination scenario. In general, thedifference between the synthesized spectrum and the spectrum of theselected illumination scenario can be reduced by increasing the numberof the light sources in a flash unit. In any case, the minimum number ofthe light sources is two.

Accordingly, the method for spectral synthesis, according to oneembodiment, comprises combining a plurality of emissive light sourcesfor providing a combined output beam; and producing the output spectrumfor the combined output beam at least partially based on a referencespectrum. The method further comprises sensing a spectrum of ambientlight for providing the reference spectrum. Alternatively, the referencespectrum is selected from a plurality of stored spectra. In oneembodiment, the stored spectra are representable by a plurality ofweighting values for said combining. In another embodiment, the methodfurther comprises: sensing a spectrum of ambient light for providing asensed spectrum; and selecting the reference spectrum at least partlybased on the sensed spectrum. In general, the plurality of emissivelight sources comprise: a first emissive light source arranged toprovide a first light beam of a first spectrum; and a second emissivelight source arranged to provide a second light beam of a secondspectrum, wherein at least part of the second spectrum is different fromthe first spectrum, and wherein at least one of the first emissive lightsource and the second emissive light source is adjustable for producingthe output spectrum. In one embodiment, at least one of the firstemissive light source and the second emissive light source is arrangedto receive a pulse-width modulated power for producing a correspondinglight beam, and wherein pulse width of the modulated power is changedfor adjusting said at least one of the first emissive light source andthe second emissive light source. In another embodiment, each of theplurality of emissive light sources is arranged to receive an electriccurrent for producing a corresponding light beam, and wherein amplitudeof the electric current received by at least one of said plurality ofemissive light sources is adjustable for producing the output spectrum.

To state it differently, the method comprises

providing a first emissive light source and a second emissive lightsource for a camera flash, wherein the first emissive light source isconfigured for producing a first light output with a first spectraldistribution, and the second emissive light source is configured forproducing a second light output with a second spectral distributiondifferent from the first spectral distribution; and providing electricalaccess to the first and second emissive light sources such that at leastthe first light output is adjustable relative to the second light outputfor producing a combined light output with a third spectraldistribution. Likewise, the apparatus, according to one embodiment,includes a first emissive light source configured for producing a firstlight output with a first spectral distribution; and a second emissivelight source configured for producing a second light output with asecond spectral distribution different from the first spectraldistribution, wherein at least the first light output is adjustablerelative to the second light output for producing a combined lightoutput with a third spectral distribution for a camera flash.

According to various embodiments, the electrical current adjustment canbe achieved by adjusting the amplitude of the current or by changing thepulse width in a pulse-width modulation. Moreover, one or more weightingvalues can be stored so that the adjustment can be based on at least onestored weighting value in order to produce the combined light outputwith the third spectral distribution.

It is possible to store a plurality of illuminant spectral distributionsso as to allow a user to select the third spectral distribution from theilluminant spectral distributions. According to various embodiments, themethod further comprises obtaining a reference spectral distribution sothat the adjustment can be at least partially based on the referencespectral distribution, wherein the reference spectral distribution isobtained by sensing the ambient light or obtained from a memory, such asa look-up table.

The method, according to various embodiments, can be carried out by asoftware program embedded in a computer readable storage medium orembedded in a processor having programming codes to carry out thevarious steps as described above.

The camera flash unit, according to various embodiments, comprises aplurality of emissive light sources for providing a combined outputbeam; and a power receiver for receiving electric current to power eachof the plurality of emissive light sources, wherein the electric currentto power at least some of the plurality of emissive light sources isadjustable so as to produce an output spectrum for the combined outputbeam at least partially based on a reference spectrum. The plurality ofemissive light sources comprise:

a first emissive light source arranged to provide a first light beam ofa first spectrum; and

a second emissive light source arranged to provide a second light beamof a second spectrum, wherein at least part of the second spectrum isdifferent from the first spectrum, and wherein at least one of the firstemissive light source and the second emissive light source is adjustablefor producing the output spectrum. The flash unit may include a batteryfor providing electrical current to the first and second emissive lightsources. The flash unit may also include

a first current source for providing electrical current to the firstemissive light source, and a second current source for providingelectrical current to the first emissive light source. The flash unitmay include a control module configured to provide electrical current toeach of the first and second emissive light sources, wherein at leastthe electrical current to the first emissive light source is adjustable.The electrical current to the first emissive light source can beprovided in a pulse-width modulation mode and the electrical current tothe first emissive light source is adjustable by changing pulse width inthe modulation mode. In a camera having an above-described flash unit,it is possible to include a look up table configured for storingweighting values to allow the control module to provide electricalcurrent to each of the first and second emissive light sources based onthe weighting values. The look up table can be configured to store aplurality of weighting values indicative of a plurality of illuminationscenarios. The camera can be a digital camera having a solid-statesensor for capturing an image formed at the image plane of a lensmodule, or a film-based camera configured for placing a section ofphotographic film at the image plane for image capturing.

An apparatus is provided which comprises a connector for receiving aflash unit, wherein the flash unit comprises a plurality of emissivelight sources arranged to receive electric current for producing acombined light output; and a processor configured to adjust the electriccurrent so as to produce an output spectrum of the combined light outputat least based on a reference spectrum. In one embodiment, the apparatuscomprises a sensor for sensing a spectrum of ambient light for providingthe reference spectrum. In another embodiment, the apparatus comprises amemory for storing data indicative of a plurality of stored spectra,wherein the reference spectrum is selected from the stored spectra. Thereference spectrum can be selected based on the sensed spectrum or by auser who determines the type of ambient light source at the time ofpicture taking. In one embodiment, the stored spectra are representableby a plurality of weighting values for producing the combined lightoutput.

In one embodiment, the apparatus comprises a first emissive light sourceconfigured for producing a first light output with a first spectraldistribution; a second emissive light source configured for producing asecond light output with a second spectral distribution different fromthe first spectral distribution, wherein at least the first light outputis adjustable relative to the second light output for producing acombined light output with a third spectral distribution for a cameraflash; electrical connectors for providing electrical access to thefirst and second emissive light sources so as to adjust at least thefirst light output, and/or a battery for providing electrical current tothe first and second emissive light sources, and/or a first currentsource for providing electrical current to the first emissive lightsource, and a second current source for providing electrical current tothe first emissive light source.

The apparatus may have a control module configured to provide electricalcurrent to each of the first and second emissive light sources, whereinat least the electrical current to the first emissive light source isadjustable, wherein the electrical current to the first emissive lightsource is provided in a pulse-width modulation mode and the electricalcurrent to the first emissive light source is adjustable by changing thepulse width in the modulation mode.

The apparatus may have a look up table configured for storing weightingvalues to allow the control module to provide electrical current to eachof the first and second emissive light sources based on the weightingvalues, wherein the look up table is configured to store a plurality ofweighting values indicative of a plurality of illumination scenarios.

The apparatus can be a stand-alone camera, or an electronic device, suchas a mobile terminal. In one embodiment, the apparatus comprises amemory for storing a software program having programming codes forcarrying out the method of producing an output spectrum of a flash unitas described above. In a different embodiment, the programming codes areembedded in a processor. In yet another different embodiment, theapparatus is configured to receive a memory unit, such as a computerreadable storage medium for storing the afore-mentioned softwareprogram.

Also provided are a camera, comprising a lens module for forming animage at an image plane; an apparatus for providing illumination; and animage forming medium for capturing the image formed at the image plane,wherein the image forming medium comprises a solid-state image sensor ora photographic film. The apparatus comprises a mobile terminal.

Briefly, a method and apparatus for spectrum synthesis in a flash unitare provided. The flash unit has two or more emissive light sources withdifferent spectral distributions. Each of the light sources can beadjusted relative to each other so that the outputs from the lightsources can be combined to mimic the spectral distribution of a selectedillumination scenario. A look-up table is used to store a plurality ofweighting values so that different weighting values can be used toproduce various synthesized spectra from the different spectraldistributions of the emissive light sources. It will be understood bythose skilled in the art that the foregoing and various other changes,omissions and deviations in the form and detail thereof may be madewithout departing from the scope of this invention.

What is claimed is:
 1. A method for producing an output spectrum of aflash unit, comprising: displaying one or more illumination scenarios,the illumination scenarios each having a particular illuminant spectraldistribution; receiving a selection of an illumination scenario; andadjusting base spectra of a plurality of light sources to create acombined output beam having an output spectrum that corresponds to theselected illumination scenario's spectral distribution.
 2. The method ofclaim 1, further comprising: determining the spectral distribution ofambient light surrounding the flash unit; comparing the spectraldistribution of the ambient light to the selected illuminationscenario's spectral distribution; and adjusting the base spectra atleast partially based on the comparison.
 3. The method of claim 2,further comprising sensing the spectral distribution of ambient light toproviding a reference spectrums.
 4. The method of claim 1, wherein theillumination scenarios include at least one of sunlight, cloudy sky,tungsten lamp, fluorescent light, and candle light.
 5. The method ofclaim 1, further comprising: sensing a spectrum of ambient light forproviding a sensed spectrum; and selecting a reference spectrum at leastpartly based on the sensed spectrum.
 6. An apparatus for producing anoutput spectrum of a flash unit, the apparatus comprising: a userinterface for displaying one or more illumination scenarios, theillumination scenarios each having a particular illuminant spectraldistribution; a unit that receives a selection of an illuminationscenario; and a control module in communication with the user interfaceand the unit, the control module adjusts spectra of a plurality of lightsources to create a combined output beam having an output spectrum thatcorresponds to the selected illumination scenario's spectraldistribution.
 7. The apparatus of claim 6, wherein the unit determinesthe spectral distribution of ambient light surrounding the flash unit.8. The apparatus of claim 7, wherein the unit compares the spectraldistribution of the ambient light to the selected illuminationscenario's spectral distribution.
 9. The apparatus of claim 8, whereinthe unit adjusts the base spectra at least partially based on thecomparison the spectral distribution of the ambient light to theselected illumination scenario's spectral distribution.
 10. Theapparatus of claim 6 further comprising: a connector for receiving theflash unit; and a processor in communication with the control module,the processor is configured, at least, to adjust electric current to thelight source.
 11. The apparatus of claim 6 further comprising a camera.12. The apparatus of claim 6 further comprising a mobile terminal. 13.The apparatus of claim 6 further comprising a sensor for sensing aspectrum of ambient light for providing a sensed spectrum, wherein areference spectrum is selected at least partly based on the sensedspectrum.